The present invention relates to a measuring apparatus for determining dimensional characteristics of a measurement object and to a related method of measuring dimensional characteristics. More particularly, the invention relates to a measuring apparatus and method exploiting color dependent foci of an optical system for determining a distance between a sensor head and a measurement point on a measurement object in a non-contact manner.
A typical measuring apparatus for determining dimensional characteristics of a measurement object is a coordinate measuring machine (CMM) comprising a sensor head that is movable relative to the measurement object along one or more orthogonal axes. The sensor head is configured to detect a number of measurement points on the measurement object. The position of the sensor head relative to the detected measurement point or measurements points is transformed into coordinates of a predefined coordinate system. Once the coordinates for a plurality of measurement points are determined, dimensional characteristics of the measurement object, such as the diameter of a bore hole, the depth of a groove or the distance between specific features on the measurement object, can be calculated.
There are various types of sensor heads for coordinate measuring machines. For some applications, non-contact sensor heads are desired. A non-contact sensor head is capable of detecting one or more measurement points without making physical contact to the measurement object.
U.S. Pat. No. 7,414,740 B2 discloses a non-contact sensor head and a method for measuring the thickness of a hot glass body. The sensor head is based on chromatic aberration and exploits the color dependent focus positions of an optical system. The optical system guides polychromatic light from a light source to the measurement object. Reflected light is conducted through the optical system into a spectrometer. The reflected light spectrum is analyzed using the spectrometer and the thickness of the glass body can be determined from the difference between the two wavelengths where the reflected light intensities in the spectrum are maximum.
Other measuring apparatus exploiting color dependent focus positions or focal shift of an optical system are disclosed by DE 197 13 362 A1, U.S. Pat. No. 5,785,651, U.S. Pat. No. 7,561,273 B2 and DE 10 2005 023 351 A1.
It seems as if the existing non-contact sensor heads each have been designed and optimized for certain measurement tasks and applications, such as determining the thickness of a hot glass body. Some designs aimed to achieve a long working distance between the sensor head and the measurement object, which is desirable for measuring deep structures on a measurement object. Other designs are directed to very specific applications, such as the measurement of the cornea of the human eye. Despite these various existing designs, there is a need for a non-contact measuring apparatus and method that can be flexibly used for determining dimensional characteristics of various measurement objects.